1. Field of Invention
The present invention relates to an electro-optical device consisting of a substrate provided with a semiconductor layer, to a method for making the electro-optical device, and to an electronic apparatus. In particular, the present invention relates to an electro-optical device in which a channel region of the semiconductor layer is connected to a capacitor line, to a method for making the electro-optical device, and to an electronic apparatus.
2. Description of Related Art
A silicon-on-insulator (SOI) technology, which includes the formation of a semiconductor layer composed of single-crystal silicon layer on an insulating substrate and of semiconductor devices such as transistor elements on the semiconductor layer, has advantages such as high-speed operation, low electrical power consumption and high-density integration of the elements. The SOI technology is applicable to electro-optical devices, for example, switching elements of a TFT array in a liquid crystal device.
In typical bulk semiconductor components, channel regions of transistor elements are maintained at a given potential by an underlying substrate. Thus, a parasitic bipolar effect generated by a change in potential in the channel region will not cause deterioration of electrical characteristics, such as a withstanding voltage.
In electro-optical devices such as liquid crystal devices, for example, transistor elements constituting switching elements in a TFT array may be completely isolated by an oxide insulating film. Hence, the channel regions in the transistor elements cannot be fixed at a given potential, and the channel regions are in an electrically floating state. In particular, when the transistor elements have a structure consisting of a single-crystal silicon layer, mobility of carriers moving in the channel may become high. Thus, collision of carriers accelerated by an electric field in the vicinity of the drain region with the crystal lattice may cause a phenomenon called impact ionization, and, for example, holes may be generated and accumulated in the bottom of the channel in an N-channel TFT. When charge is accumulated in the channel, the NPN structure of the TFT (in the case of the N-channel type) operates as an apparent bipolar element, resulting in deterioration of electrical characteristics, such as the source/drain withstanding voltage, of the element due to an extraordinary current. A series of phenomena caused by electrical floating of the channel regions are called substrate stray effects.
The present invention provides an electro-optical device which prevents deterioration of source/drain withstanding voltage by the substrate stray effects of transistor elements composed of a single-crystal silicon layer covered with an insulating film, and which can stabilize and improve electrical characteristics of the elements, a method for making the electro-optical device, and an electronic apparatus.
An electro-optical device in accordance with the present invention may consist of, on a substrate, a plurality of scanning lines, a plurality of data lines crossing the plurality of scanning lines, transistors connected to each of the scanning lines and to each of the data lines, pixel electrodes connected to the transistors, and storage capacitors, wherein an extending portion of a semiconductor layer functioning as a channel region of the transistor is connected to a capacitor line functioning as an electrode of the storage capacitor.
According to such a configuration in the present invention, the channel region of the semiconductor layer composed of a single-crystal silicon layer is connected to the capacitor line as an electrode of a storage capacitor, the channel region is maintained at a potential of the capacitor line, an extraordinary current in the transistor element is avoided, and electrical characteristics of the element are stabilized.
In the electro-optical device of the present invention, the extending portion and the capacitor line may be connected to each other by a connecting line via a first contact hole formed above the extending portion and a second contact hole formed on the capacitor line, and the scanning line and the capacitor line may lie in the same layer and have detour sections formed so as to detour around the first contact hole.
According to such a configuration of the present invention, the channel region of the semiconductor layer can be connected to the capacitor line while effectively using limited spaces. Since the connecting line and the contact holes can be simultaneously formed together with the data line, these can be formed using conventional production processes. Thus, in the electro-optical device of the present invention, the connecting line and the data line are preferably formed on the same layer.
In the electro-optical device of the present invention, the thickness of the semiconductor layer may lie in a range of 100 to 180 nm.
According to such a configuration of the present invention, the thickness of the semiconductor layer is greater than 100 nm. Thus, when the contact hole for connecting the pixel electrode to the drain region of the semiconductor layer, the contact hole will not pass through the semiconductor layer. Since the thickness of the semiconductor layer is less than 180 nm, the bumps of the device substrate due to the thickness of the semiconductor layer can be suppressed as much as possible. As a result, disclination when the liquid crystal is aligned can be suppressed and the display quality can be maintained at a satisfactory level.
In the electro-optical device of the present invention, a gate insulating film having a thickness of 450 nm to 650 nm may be inserted between the channel region of the semiconductor layer and a gate electrode region of the scanning line.
Since the thickness of the gate insulating film is greater than 450 nm according to such a configuration of the present invention, the liquid crystal can be driven by a required electrical power voltage without dielectric breakdown. Since the thickness of the gate insulating film is less than 650 nm, the gate capacitance can be increased so that the operating speed of the TFT elements, which is essential for driving the liquid crystal display device, can be ensured.
In the electro-optical device of the present invention, the impurity concentration at the edge of the channel region of the semiconductor layer may be higher than the impurity concentration of the other parts of the channel region.
Since the impurity concentration at the edge of the channel region of the semiconductor layer is higher than the impurity concentration of the other parts of the channel region according to such a configuration of the present invention, the apparent threshold voltage in this region is high. Thus, a leakage current can be prevented when the electric field from the gate electrode is concentrated in the edge of the channel region of the semiconductor layer.
In the electro-optical device, the thickness of the scanning line may lie in a range of 350 nm to 700 nm.
Since the thickness of the scanning line is greater than 350 nm according to such a configuration of the present invention, the wiring resistance can be reduced and a decreased in the writing rate of signals into pixels due to wiring delay can be sufficiently suppressed. Since the thickness of the scanning line is less than 550 nm, the bumps of the device substrate due to the thickness of the scanning line can be suppressed as much as possible. As a result, disclination when the liquid crystal is aligned can be suppressed and display quality can be maintained at a satisfactory level.
In the electro-optical device, the scanning line may consist of a polysilicon layer or at least two layers of a polysilicon and a conductive metal layer. According to such a configuration of the present invention, the conductivity can be enhanced; hence, a decrease in the writing rate of signals into pixels due to wiring delay can be sufficiently suppressed. In particular, the conductivity can be further enhanced in a scanning line composed of a polysilicon layer and a conductive metal layer. Thus, the formed scanning line has reduced wiring delay when the thickness is reduced, and the bumps of the device substrate due to the thickness can be suppressed as much as possible. As a result, disclination when the liquid crystal is aligned can be suppressed and display quality can be maintained at a satisfactory level.
In the electro-optical device of the present invention, an interlayer insulating layer having a thickness of 800 nmxc2x1200 nm may be inserted between the data lines and at least the scanning lines.
Since the thickness of the interlayer insulating layer is greater than 600 nm according to such a configuration of the present invention, capacitor coupling between the scanning line and the data line can be suppressed as much as possible and deterioration of writing characteristics of signals into pixels can be prevented. Since the thickness of the interlayer insulating layer is less than 1,000 nm, the throughput in the deposition step of the interlayer insulating layer can be improved.
In the electro-optical device of the present invention, the thickness of the data lines may lie in a range of 350 nm to 700 nm.
Since the thickness of the data line is greater than 350 nm according to such a configuration of the present invention, the wiring resistance can be reduced, and thus a decreased in the writing rate of signals into pixels due to wiring delay can be sufficiently suppressed. Since the thickness of the data line is less than 700 nm, the bumps of the device substrate due to the thickness of the data line can be suppressed as much as possible. Thus, disclination when the liquid crystal is aligned can be suppressed and display quality can be maintained at a satisfactory level.
In the electro-optical device of the present invention, an interlayer insulating layer having a thickness of 800 nmxc2x1200 nm may be inserted between the data lines and at least the pixel electrode.
Since the thickness of the interlayer insulating layer is greater than 600 nm according to such a configuration of the present invention, capacitance coupling between the data line and the pixel electrode can be suppressed as much as possible, and deterioration of writing characteristics of signals into pixels can be prevented. Since the thickness of the interlayer insulating layer is less than 1,000 nm, the throughput in the deposition step of the interlayer insulating layer can be improved.
The electro-optical device of the present invention may consist of a lightshielding layer provided between the substrate and the semiconductor layer.
According to such a configuration of the present invention, direct light incident on the rear surface of the substrate and light reflected by the rear surface of the substrate do not enter the transistor element forming regions. Thus, deterioration of writing characteristics of signals into pixels due to light leakage can be prevented.
In the electro-optical device of the present invention, the thickness of the light-shielding layer may lie in a range of 200 nm to 400 nm.
Since the thickness of the light-shielding layer is greater than 200 nm according to such a configuration of the present invention, the leakage current generated by the light reflected by the rear surface of the substrate can be suppressed to a level which does not affect writing characteristics into pixels. Since the thickness of the light-shielding layer is less than 400 nm, the bumps of the device substrate due to the thickness of the light-shielding layer can be suppressed as much as possible. As a result, disclination when the liquid crystal is aligned can be suppressed and display quality can be maintained at a satisfactory level.
A method for making an electro-optical device of the present invention may consist of forming a semiconductor layer functioning as a channel region, an extending portion of the channel region, and one electrode of a storage capacitor on a substrate, forming an insulating film on the semiconductor layer, forming a scanning line and a capacitor line functioning as another electrode of the storage capacitor on the insulating film, and connecting the extending portion to the capacitor line.
According to such a configuration of the present invention, the channel region and the capacitor line of the semiconductor layer are connected to each other. Thus, the channel region is fixed to a potential of the capacitor line, and problems, such as deterioration of the source-drain withstand voltage of the transistor element due to substrate stray effects which are produced by the SOI structure will not occur. The produced electro-optical device can have elements with stabilized electrical characteristics.
In the method for making an electro-optical device of the present invention, in the step for connecting the extending portion to the capacitor line, the extending portion and the capacitor line may be connected by a connecting line via a first contact hole formed on the extending portion and a second contact hole formed on the capacitor line, and a data line may be formed so as to be connected to the semiconductor layer via a third contact hole formed on the semiconductor layer.
According to such a configuration of the present invention, the connecting line and the data line can be simultaneously formed of the same material. Thus, the connecting line can be formed without additional steps. The method for making an electro-optical device of the present invention may further consist of, prior to forming the semiconductor layer, forming a light-shielding layer on the substrate at least at a position corresponding to the semiconductor layer.
According to such a configuration of the present invention, direct light incident on the rear surface of the substrate and light reflected by the rear surface of the substrate do not enter the transistor element forming regions. Thus, the produced electro-optical device does not cause deterioration of writing characteristics of signals into pixels due to light leakage.
In the method for making an electro-optical device of the present invention, forming the semiconductor layer may consist of bonding a single-crystal silicon substrate onto the substrate, and removing unnecessary parts of the bonded single-crystal silicon substrate to form another semiconductor layer consisting of single-crystal silicon.
In the method for making an electro-optical device of the present invention, the thickness of the semiconductor layer may lie in a range of 100 to 180 nm.
According to such a configuration of the present invention, the thickness of the semiconductor layer is greater than 100 nm. Thus, when the contact hole for connecting the pixel electrode to the drain region of the semiconductor layer, the contact hole will not pass through the semiconductor layer. Since the thickness of the semiconductor layer is less than 180 nm, the bumps of the device substrate due to the thickness of the semiconductor layer can be suppressed as much as possible. As a result, disclination when the liquid crystal is aligned can be suppressed and the display quality can be maintained at a satisfactory level.
In the method for making an electro-optical device of the present invention, in forming the insulating film, an n-type impurity may be implanted into a P channel of the semiconductor layer at a dosage of 1e11/cm2 to 4e11/cm2.
According to such a configuration of the present invention, the threshold voltage, which is an important switching property of TFT elements necessary for driving the liquid crystal device, can be appropriately controlled within an optimum practical range from xe2x88x921.0 to xe2x88x922.0 volts.
In the method for making an electro-optical device of the present invention, in forming the insulating film, a p-type impurity may be implanted into an N channel of the semiconductor layer at a dosage of 5e11/cm2 to 15e11/cm2.
According to such a configuration of the present invention, the threshold voltage, which is an important switching property of TFT elements necessary for driving the liquid crystal device, can be appropriately controlled within an optimum practical range from 1.0 to 2.0 volts.
The method for making an electro-optical device of the present invention may further consist of, prior to forming the insulating film, forming a gate insulating film on the semiconductor layer. The method for making an electro-optical device further consists of, subsequent to forming the insulating film, forming a gate insulating film on the semiconductor layer. This makes it possible to control the threshold voltage.
The method for making an electro-optical device of the present invention may further consist of, subsequent to forming the insulating film, implanting an n-type impurity into the channel edge of the P channel of the semiconductor layer and p-type impurity into the channel edge of the N channel of the semiconductor layer at a dosage of two to ten times that of the impurity implanted into the entire channel region.
According to such a configuration of the present invention, the impurity concentration at the channel edge of the semiconductor layer is higher than that of other portions in the channel region. Since the apparent threshold voltage is higher in this region, a leakage current does not flow when an electric field from the gate electrode is concentrated at the channel edge of the semiconductor layer.
In the electro-optical device of the present invention, the thickness of the scanning line may lie in a range of 350 nm to 700 nm.
In the method for making an electro-optical device of the present invention, in forming the scanning line and the capacitor line, a p-type impurity may be implanted into the P channel of the semiconductor layer at a dosage of 2e13/cm2 to 1e14/cm2 to form a lightly doped drain (LDD) region and a p-type impurity may be implanted into the semiconductor layer at a dosage of 5e14/cm2 to 2e15/cm2 to form a source/drain region.
According to such a configuration of the present invention, the electric field intensity has a moderate distribution in the vicinity of the drain by the presence of the LDD region. Thus, the withstanding voltage of the transistor element can be maintained at 10 volts or more which is an electrical source voltage necessary for driving the liquid crystal device. Since the sheet resistance and the contact resistance in the source and drain regions can be sufficiently suppressed, a decrease in the ON current due to parasitic resistance of the transistor elements can be suppressed.
In the method for making an electro-optical device of the present invention, in forming the scanning line and the capacitor line, an n-type impurity may be implanted into the N channel of the semiconductor layer at a dosage of 6e12/cm2 to 2.5e13/cm2 to form a lightly doped drain (LDD) region and an n-type impurity may be implanted into the semiconductor layer at a dosage of 1e15/cm2 to 4e15/cm2 to form a source/drain region.
According to such a configuration of the present invention, the electric field intensity has a moderate distribution in the vicinity of the drain by the presence of the LDD region. Thus, the withstanding voltage of the transistor element can be maintained at 10 volts or more which is an electrical source voltage necessary for driving the liquid crystal device. Since the sheet resistance and the contact resistance in the source and drain regions can be sufficiently suppressed, a decrease in the ON current due to parasitic resistance of the transistor elements can be suppressed.
In the method for making an electro-optical device of the present invention, an activation annealing treatment is performed at a temperature in a range of 800xc2x0 C. to 900xc2x0 C. subsequent to forming the scanning line and the capacitor line.
According to such a configuration of the present invention, impurities implanted in the LDD region and the source and drain regions can be activated. When the temperature is less than 800xc2x0 C., the implanted impurities cannot be activated. When the temperature is greater than 900xc2x0 C., the impurities are significantly diffused in the transverse direction during the annealing treatment and the impurity profile having the LDD structure which is necessary for securing the withstand voltage of the transistor elements cannot be formed.
In the method for making an electro-optical device of the present invention, the capacitor line and a scanning line may be simultaneously formed in connecting the extending portion to the capacitor line.
According to such a configuration of the present invention, the production process can be simplified.
The method for making an electro-optical device of the present invention may consist of forming a first contact hole connected to the extending portion and a second contact hole connected to the capacitor line, and forming a connecting line for connecting the first contact hole to the second contact hole.
According to such a configuration of the present invention, the extending portion and the capacitor line can be connected to each other without additional steps.
In the method for making an electro-optical device of the present invention, the connecting line and a data line may be simultaneously formed.
According to such a configuration of the present invention, the connecting line can be formed without additional steps.
The electro-optical device of the present invention may further consist of a second substrate opposing the surface of the substrate provided with the semiconductor layer, and a liquid crystal disposed between the two substrates and driven by transistor elements formed in the semiconductor layer.
An electronic apparatus of the present invention may consist of a light source, the above described electro-optical device for modulating light emitted from the light source in response to image information, and a projection device for projecting the light modulated by the electro-optical device.